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But he warned a Trump victory could gravely damage the Republican party

West Palm Beach, Florida (CNN)On the eve of the Florida primary, a reflective Marco Rubio looked out at a roaring crowd of diverse supporters and, once again, expressed regret.

"The fact that I'm here now in front of those cameras comes with responsibility, and I'll confess, I've learned that myself," the Florida senator said. "A few weeks ago, in response to some attacks about the size of my ears and something else, I responded in kind with an attack about -- I'm not going to repeat it -- stuff."

Rubio talks to reporters in June after a closed door briefing on Capitol Hill.

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Photos:Moments from Marco Rubio's career

Rubio speaks to the media in front of a wall dedicated to the victims of the violence in Venezuela as he shows support for the Venezuelan community at a restaurant in Doral, Florida, in April 2014.

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Rubio speaks to the media at the Doral restaurant in April 2014.

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Rubio checks his phone in March as he arrives for a Capitol Hill news conference to introduce a proposal for an overhaul of the tax code.

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Rubio, left, is greeted as he arrives at a restaurant in Doral in February 2014.

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Rubio, left, and Florida Gov. Rick Scott speak to the media in Doral in February 2014.

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From left, Sens. Cory Booker, Rubio, Kristen Gillibrand and John McCain wait for Obama to deliver the State of the Union address in January 2014.

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Rubio addresses an event held by the American Enterprise Institute for Public Policy Research in January 2014.

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Paralympian Brad Snyder poses with Rubio during a Team USA Congressional visit in November 2013.

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Rubio speaks in November 2013 during a campaign stop for Virginia Attorney General Ken Cuccinelli, the Republican candidate for governor of Virginia.

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Rubio confers with McCain as U.S. Ambassador to Syria Robert Ford testifies before the Senate Foreign Relations Committee in October 2013.

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Rubio, center, speaks to members of the media as Sen. Ron Johnson, left, and Rep. Ron DeSantis listen during a news conference on Capitol Hill in October 2013.

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Rubio returns to the Capitol after meeting with Obama and other Republican leaders about the government shutdown in October 2013.

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Rubio speaks to an aide on Capitol Hill as he arrives for the weekly Senate Republican Policy Committee luncheon in September 2013.

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"I felt terrible about it," Rubio said. "I realized that win or lose, there are people out there that see what I'm doing and follow it as a role model."

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After days of unflinching optimism on the campaign trail, the gravity of Tuesday's Florida primary appeared to bear down at one of Rubio's last stops of the day. And potentially, one of the last stops of his presidential campaign.

If Rubio can't defy the latest polls and rise to victory here, it could help pave the way for Trump to clinch the nomination and anoint a brand of politics than many Republicans privately say they have come to loathe and even fear.

"Leadership is not about going to angry and frustrated people and saying, 'You should be even angrier and more frustrated, and you should be angry and frustrated at each other,'" Rubio said. "That is not leadership. You know what that is? That's called demagoguery, and it is dangerous."

He lamented Trump's over-the-top style, from his use of profanity -- "we have never had a presidential candidate that has to be bleeped out" -- to his apparent willingness to pay the legal fees for a supporter who sucker punched a protester.

Photos:The week in politics

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Donald Trump supporter Birgitt Peterson of Yorkville, Illinois, argues with protesters outside the UIC Pavilion after the canceled rally for the Republican presidential candidate in Chicago on Friday, March 11. Peterson told the Chicago Tribune that she responded with the Nazi-style salute after anti-Trump protestors called her a Nazi. The Trump rally was canceled because of concerns after hundreds of protestors packed into the University of Illinois at Chicago venue.

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Ron Reagan and sister Patti Davis pause at the casket of their mother, Nancy Reagan, at her gravesite at the Ronald Reagan Presidential Library in Simi Valley, California, on Friday, March 11. Funeral services for the former first lady were held at the library Friday.

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Republican presidential candidates Sen. Marco Rubio, Donald Trump, Sen. Ted Cruz and Ohio Gov. John Kasich debate on the campus of the University of Miami on Thursday, March 10, in Coral Gables, Florida. The candidates continue to campaign before the March 15 primaries in Florida and elsewhere.

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President Barack Obama waves onstage with Texas Tribune editor-in-chief Evan Smith during a keynote session at the 2016 South by Southwest festival in Austin, Texas, on Friday, March 11.

Republican presidential candidate Donald Trump is escorted off the stage by Secret Service agents after a man jumped a barrier and tried to rush the stage during a campaign rally at Dayton International Airport in Ohio on Saturday, March 12.

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Florida Sen. Marco Rubio smiles with a lipstick mark from supporter Jan Mickelsen at a campaign rally in Idaho Falls, Idaho, on Sunday, March 6.

Democratic presidential candidate Hillary Clinton takes selfies with supporters before speaking during a rally at Cuyahoga Community College in Cleveland on Tuesday, March 8.

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Republican presidential candidate Sen. Ted Cruz has makeup applied during a break at a town hall-style interview with Fox News host Megyn Kelly at Calvary Baptist Church in Raleigh, North Carolina, on Tuesday, March 8.

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Treasury Secretary Jack Lew testifies during a Senate Financial Services and General Government Subcommittee hearing on the department's 2017 budget request, on Tuesday March 8.

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President Barack Obama toasts Canadian Prime Minister Justin Trudeau during a state dinner in the East Room of the White House on Thursday, March 10.

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Voters, some of whom waited more than five hours, stand in a line that stretched well over one-half mile to get into a Democratic caucus location at Deering High School, on Sunday, March 6, in Portland, Maine. Several thousand voters showed up to choose between Hillary Clinton and Bernie Sanders.

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Lynel Zimmerman holds her baby, James Thompson, as Marine One takes off from the South Lawn of the White House with President Barack Obama aboard in Washington, on Friday, March 11.

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"I know there are people that like this stuff because he says what they want to be able to say," Rubio said. "Presidents can't say whatever they want to say. You have to be honest, you have to be correct and you have to be truthful. But you can't say whatever you want to say."

The crowd cheered with approval.

Rubio's rally at Palm Beach Atlantic University, a Christian school in South Florida, drew the kind of crowd establishment Republicans have hoped to build into a general election coalition. It was young. It was diverse. Afterwards, college students, suburban women and immigrants all crushed in around the candidate shouting, "Marco! Marco!" and vied for autographs and pictures.

Rubio opened his speech by recalling that 11 months ago this week, he announced his presidential bid at Miami's Freedom Tower, using the iconic site to highlight his immigrant roots and the significance of the American Dream.

Monday night, he made an overt plea for voters to support him -- and an unspoken one, imploring them to stop Trump.

As Luis Jimenez emerged from the scrum, he said he was still holding out hope that Rubio could notch a win in Florida. Both he and his wife had already cast early ballots for him.

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"He's used to being an underdog," Jimenez, a 44-year-old Boynton Beach resident, said. "There's a long shot."

As he gamed out Rubio's odds, Rossaida Jimenez, 43 years old, shook her head in disagreement.

"He's more positive than I am," she said.

If Rubio doesn't succeed on Tuesday and Trump wins the GOP nomination, both said they'll cast their ballots for the billionaire in November.

Later Monday, Rubio's bus pulled up to a crowd of hundreds gathered on a brightly lit, outdoor basketball court in West Miami, where Rubio would hold his potentially last rally as a presidential candidate. He hopped up on the bed of a pick-up truck and used a bullhorn to address the enthusiastic crowd, as a giant American flag hung on a building behind him.

It was a homecoming of sorts, given that Rubio said he used to play basketball in this park and it's an area where he campaigned to run for city commissioner two decades ago.

"And this is the park I wanted to be in tonight, on the eve of the most important election in a generation, in the state that always makes the difference, in the part of the state that's going to carry us over the top," he said, talking about Tuesday's primary.

Speaking more in Spanish than in English, Rubio thanked the community for their support over the years. He had fun joking around with the crowd -- especially in Spanish -- and appeared more relaxed and at ease than he has in days.

"No matter where I'll go or where I'll be, I will always be a son of this community," he told them. "I will always carry with me the hopes and dreams of generations who made possible the hopes of mine."